1. Field of the Invention
The present invention relates to coherent optical systems and in particular a transmitter and transceiver for use in coherent optical systems.
2. Description of Related Art
Any optical system involving the transmission and reception of optical signals must cope with the problems introduced by signal degradation over the transmission medium. Some methods of improving the quality of signal reception are well known. It is recognized, for example, that coherent optical systems, with optical receivers using heterodyne or homodyne techniques in particular, offer a considerable improvement over standard direct intensity systems. Conventionally, such coherent systems require a strong reference signal to be mixed with an incoming modulated signal in a receiver. Generally, this reference signal is provided by a secondary laser, or local oscillator, in the receiver itself. Additionally, for homodyne reception, the reference signal must be precisely phase locked with the incoming modulated signal. With one laser to originate the transmission and another to act as local oscillator, the difficulties involved in achieving this phase locking are severe.
Published European Patent Application EP 168192 A2 discloses a coherent transmission system in which a single laser transmitter is used to provide both modulated and reference signals. Both signals are sent together in orthogonal polarisations. At a receiver, the reference signal is divided off and separately amplified to drive a coherent detector. This system suffers from problems caused by birefringence in the transmission medium (e.g. an optical fibre) which may introduce polarisation dependent phase shifts in the two signals.